Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technology to transfer a fine pattern required for forming an integrated circuit made of a fine pattern on e.g. a Si substrate. However, the conventional photolithography method is approaching its limit, while miniaturization of semiconductor devices has been accelerated. In the case of the photolithography method, the resolution limit of a pattern is at a level of ½ of the exposure wavelength, and even when a liquid immersion method is employed, it is said to be at a level of ¼ of the exposure wavelength. Accordingly, even when an immersion method by an ArF laser (193 nm) is employed, the resolution limit is expected to be at a level of 45 nm. Therefore, as an exposure technique with 45 nm or shorter, EUV lithography is expected to be promising, which is an exposure technique employing EUV light having a wavelength further shorter than the ArF laser. In this specification, EUV light means light having a wavelength in a soft X-ray region or in a vacuum ultraviolet region, and specifically, it means light with a wavelength of from about 10 to 20 nm, particularly at a level of 13.5 nm±0.3 nm.
EUV light is likely to be readily absorbed by all materials, and the refractive indices of the materials at this wavelength are close to 1, whereby it is not possible to use a refracting optical system as used in a conventional photolithography employing visible light or ultraviolet light. Accordingly, in the EUV lithography, a reflecting optical system i.e. a reflective photomask and a mirror are used.
A mask blank is a laminate prior to patterning, to be used for the production of a photomask.
In the case of an EUV mask blank, it has a structure wherein a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light are formed in this order on a substrate of e.g. glass. As the reflective layer, a multilayer reflective film is usually employed wherein a high refractive layer and a low refractive layer are alternately laminated to improve the light reflectance when the layer surface is irradiated with EUV light. For the absorber layer, a material having a high absorption coefficient for EUV light, specifically e.g. a material containing Cr or Ta as the main component, is employed.
Patent Document 1 discloses that a nitride of a tantalum/boron alloy (TaBN), an oxide of a tantalum/boron alloy (TaBO) and an oxynitride of a tantalum/boron alloy (TaBNO) are preferred as a material for the absorber layer, since they have not only a high absorption coefficient for EUV light but also a low reflectance of a far ultraviolet light in a wavelength region (190 nm to 260 nm) of the light for inspection of a pattern.
Further, Patent Documents 1 and 2 disclose that the crystalline state of the absorber layer is preferably amorphous in order to make the absorber layer surface to be a surface excellent in smoothness, and in order to make the crystalline state of the TaBN film, the TaBO film and the TaBNO film to be amorphous, the content of B in these films is preferably from 5 to 25 at % (atomic percent).
Further, in Patent Document 3, a TaN film is formed by an ion beam sputtering method, and the stress adjustment is carried out by using xenon (Xe) as the sputtering gas.
Patent Document 1: JP-A-2004-6798 (U.S. Pat. No. 7,390,596 and U.S. Patent Application Publication No. 2008/0248409)
Patent Document 2: JP-A-2004-6799 (U.S. Pat. No. 7,390,596 and U.S. Patent Application Publication No. 2008/0248409)
Patent Document 3: U.S. Patent Application Publication No. 2006/0008749